1. Technical Field
The present invention relates to a surface light source device, and specifically to a surface light source device that is used as a backlight of a liquid crystal display.
2. Related Art
FIG. 1A is a view illustrating a surface light source device of a conventional example when the surface light source device is viewed from a lower surface side. FIG. 1B is a side view illustrating the surface light source device of the conventional example. In a surface light source device 11, a plurality of light sources 12 are disposed while facing a light incident surface 14 of a light guide plate 13. The light guide plate 13 is formed into a flat-plate shape using a high-refractive-index transparent resin, and a plurality of spherical light exit patterns 17 are projected on an opposite surface (a lower surface 16) of a light exit surface 15. For example, Japanese Unexamined Patent Publication No. 11-281976 discloses such a surface light source device.
FIGS. 2A and 2B illustrate the case where a maximum inclined angle of the light exit pattern 17 is decreased in the surface light source device 11 in FIGS. 1A and 1B. As used herein, the maximum inclined angle of the light exit pattern refers to a largest inclined angle in inclined angles of tangents drawn to the surface of the light exit pattern in a section passing through a center axis of the light exit pattern or a section parallel to the section passing through the center axis of the light exit pattern. For example, in the spherical light exit pattern in FIGS. 1A and 1B and FIGS. 2A and 2B, the maximum inclined angle is an inclined angle of the tangent drawn to an end of the light exit pattern 17 with respect to a horizontal surface. As illustrated in FIGS. 2A and 2B, for the small maximum inclined angle of the light exit pattern 17, the light incident to the light guide plate 13 from the light source 12 is hardly scattered in each direction by the light exit pattern 17, and therefore a light mixing effect is insufficiently obtained. Therefore, a portion in front of the light source 12 brightens up, a dark portion D is generated in a region between the light sources 12, and a variation in luminance is generated near the light source 12. At the same time, the light exit pattern 17 has high intensity.
FIGS. 3A and 3B illustrate the case where the maximum inclined angle of the light exit pattern 17 is increased in the surface light source device 11 in FIGS. 1A and 1B. As illustrated in FIGS. 3A and 3B, for the large maximum inclined angle of the light exit pattern 17, the light incident to the light guide plate 13 from the light source 12 is easily scattered in each direction by the light exit pattern 17, and therefore the light mixing effect is enhanced. Therefore, the variation in luminance is hardly generated near the light source 12. At the same time, when the maximum inclined angle of the light exit pattern 17 is increased to an extent that the variation in luminance is inconspicuous, an aspect ratio of the light exit pattern 17 is increased to decrease the intensity. In the case where the surface light source device is used as a liquid crystal display device, the light exit pattern 17 may possibly be crushed when the surface of the liquid crystal display device is pressed by a finger.
Accordingly, there is a practical problem in both the cases where the maximum inclined angle of the light exit pattern 17 is increased and decreased.